This is a major problem we had with the Sideways world: it created David Shepard as a character, just as a way to fake out viewers, and then did away with him as soon as the finale revealed more about this universe.
Jack: Where are we? Christian: This is a place you all made together so you could find one another...Nobody does it all alone. You needed them and they needed you. Jack: For what? Christian: To remember and to let go.
You're not John Locke, you disrespect his memory by wearing his face, but you are nothing like him. It turns out he was right about just about everything. I just wish I could have told him that when he was still alive.